Performances of vehicle batteries deteriorate over time. Thus, in some cases, a jump start needs to be carried out for starting the vehicle with deteriorated battery by temporarily connecting the deteriorated battery with an external power source. In this case, for starting the engine, a power supply to a starter is supplemented by a power source, such as a battery of another vehicle.
For example, as disclosed in JP 2001-107768 A, by connecting the battery of another vehicle with the battery of the subject vehicle using a booster cable, the electric power from the battery of another vehicle can be supplied to the starter of the subject vehicle for starting the engine.
When carrying out a jump start to the subject vehicle, a high voltage external power source, such as a battery voltage of 24 volts, which is usually used in a truck, may be connected to the subject vehicle. Suppose that the 24 volt battery voltage is still supplied to the subject vehicle after the engine start. In this case, when the electric power generation device of the subject vehicle starts an electric power generation operation, an abnormal current may flow through the electric power generation device and the abnormal current may cause damages to the electric power generation device. Suppose that the 24 volt battery voltage is removed from the subject vehicle after the engine start and the electric power generation operation of the electric power generation device is in a deactivated state at a removal time of the 24 volt battery voltage. In this case, since the external power source is removed from the battery, the battery voltage output may be decreased and an engine stall may occur caused by the voltage decrease at the battery terminals.